Non-aqueous secondary cells including a lithium-ion secondary cell have high energy density, high voltage and high capacity, and are therefore utilized widely as a power source of a variety of portable equipments. In recent years, such secondary cells are increasingly used for medium- and large-sized equipments including power tools such as electric tools, electric cars, and electric assist bicycles.
In particular, along with the spread of electronic devices such as portable phones and note-type personal computers in which reduction in size and increase in functions are in progress, there is a demand for an improvement in capacity of a secondary cell to be used for such devices. As a measure, an electrode active material showing high charge/discharge capacity has been studied and developed. In particular, as an ingredient for a negative electrode active material, a substance such as silicon and silicon oxide having large initial capacity is attracting attention as an alternative for a carbon material, such as graphite, which has been used in conventional non-aqueous secondary cell (see Patent Literatures 1 and 2).
As such a material, silicon oxide (SiOx) is gathering attention. Since this compound exhibits large volume expansion and contraction accompanying a charge/discharge reaction, it is known to suffer from problems such as increase in irreversible capacity caused by gradual pulverization of particles during each charge/discharge cycle of the cell and a reaction between Si precipitated on the surface with a non-aqueous electrolyte. As a result, the cell may exhibits a so-called cycle performance reduction phenomenon in which the capacity is reduced by repeated charging and discharging. In order to suppress the cycle performance reduction, a variety of studies has been carried out. For example, use of a composite material of SiOx and a carbon material (Patent Literature 3), or use of graphite particles satisfying specific requirements as a carbon material (Patent Literature 4) has been proposed. However, the industry demands for further improvements in cell capacity (Ah) and charge/discharge capacity performances (cycle performances) upon repeated charging and discharging.
There is also a demand for an electrode active material which maintains a constant capacity even at a high charge/discharge rate (i.e., having a high load characteristic). In order to achieve the high load characteristic, the electrode active material needs to have high conductivity.